Light emitting diode component

ABSTRACT

The present invention relates to a light emitting diode component, comprising a light emitting semiconductor structure having a top surface, and a micro-optical multilayer structure arranged to guide light out from said light emitting semiconductor structure, said micro-optical multilayer structure comprising a plurality of layers, wherein an i+1:th layer is arranged on top an i:th layer in a sequence as seen from said semiconductor structure, wherein a refractive index, ni, of the i:th layer is greater than a refractive index, ni+l, of the i+1:th layer, and wherein a thickness of the i+1:th layer is greater than a thickness of the i:th layer. The present invention also relates to a light emitting diode comprising such a light emitting diode component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/786,947 filed Oct. 23, 2015, entitled “A LIGHT EMITTING DIODECOMPONENT”, issuing as U.S. Pat. No. 9,966,511 on May 8, 2018, which isa § 371 application of International Application No. PCT/EP2014/057939filed Apr. 17, 2014, which claims the benefit of European PatentApplication No. 13165312.3 filed Apr. 25, 2013. U.S. patent applicationSer. No. 14/786,947, International Application No. PCT/EP2014/057939 ,and European Patent Application No. 13165312.3 are incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates generally to a light emitting diodecomponent, and in particular to a light emitting diode componentcomprising a micro-optical multilayer structure used to improve theperformance of the light emitting diode component.

BACKGROUND OF THE INVENTION

Semiconductor based light emitting diodes (LEDs) are among the mostefficient light sources available today. LEDs provide longer lifetime,higher photon flux efficacy, lower operating voltage, narrow-band lightemission, and flexibility in terms of assembly compared to conventionallight sources.

GaN based LEDs are for instance successfully employed as high powerlight sources in solid state lighting applications such as illumination,traffic lighting, indoor/outdoor displays, and backlighting electronicdisplays.

It is however challenging to efficiently couple light out of the LEDs.An epitaxial layer normally has a high refractive index compared to air,n_(air)=1, or glass, n_(glass)=1.5. The refractive index of GaN,n_(GaN), is for example in the range 2.3-2.5 at visible wavelengths. Thelarge refractive index mismatch between the epitaxial layer and asurrounding medium causes a large portion of the light generated withinthe epitaxial layer to be scattered or reflected at the interfacebetween the epitaxial layer and its surrounding medium. Only lighttravelling at angles within a relatively narrow escape cone associatedwith the interface can refract into the surrounding medium and escapethe epitaxial layer. In other words, the external quantum efficiency ofthe LED is low resulting in a reduced brightness of the LED.

The situation is even more complex when it comes to white lightgeneration using LEDs. White light is generally not generated bytailoring of the emission energy of the epitaxial layer. Instead, lightfrom a blue epitaxial layer, typically GaN based, is converted to whitelight with the aid of a yellow phosphor material surrounding theepitaxial layer. The surrounding phosphor material down-converts asubstantial portion of the epitaxial layer's blue light, changing itscolor to yellow. Hence, the LED emits both blue and yellow light, whichin combination provide white light. In another approach light from aviolet or ultraviolet emitting epitaxial layer has been converted towhite light by surrounding the epitaxial layer with multicolorphosphors.

The phosphor material, which is typically deposited on a substrate onwhich the epitaxial layer is grown , causes additional light losses dueto scattering at the interface between the substrate and the phosphormaterial as well as back scattering of light at for instance anair/phosphor material interface.

In order to improve the performance of the LEDs there is therefore aneed for better efficiency in guiding light emitted from the epitaxiallayer out from the LED.

SUMMARY OF THE INVENTION

An object of the present invention is to solve or at least to reduce theproblems discussed above.

In particular according to a first aspect of the invention a lightemitting diode component is provided, said light emitting diodecomponent comprising a light emitting semiconductor structure having atop surface and a micro-optical multilayer structure arranged to guidelight out from said light emitting semiconductor structure, saidmicro-optical multilayer structure comprising a plurality of layers,wherein an i+1:th layer is arranged on top an i:th layer in a sequenceas seen from said semiconductor structure, wherein a refractive index,n_(i), of the i:th layer is greater than a refractive index, n_(i+1), ofthe i+1:th layer. The value of i is selected from the set of positiveintegers, thus i can be 1, 2, 3, 4, etc. Hence, the invention exploitslocal index matching techniques to reduce light trapping within thelight emitting diode component providing improved light guidance andlight output. Another advantage being that the need for a bulkyencapsulation of the light emitting diode component, using for instancea dome lens, is reduced. This, in turn, reduces the volume size of theemitter as well as its related costs.

According to an embodiment of the present invention a thickness of thei+1:th layer is larger than a thickness of the i:th layer. This reducesthe total internal reflection which increases the light extractionefficiency and thus improves the light output performance. In anembodiment the thickness of the i+1:th layer is 10% larger than thethickness of the i:th layer. In an embodiment the thickness increase ofthe i+1:th layer with respect to the i:th layer depends on the relativeincrease of the refractive index of the i+ 1 :th layer with respect tothe i:th layer.

According to an embodiment of the present invention a thickness of thei:th layer, t_(i), is given by:

$t_{i} = \frac{\sqrt{A}}{\tan \left\lbrack {\sin^{- 1}\left( \frac{n_{i + 1}}{n_{i}} \right)} \right\rbrack}$

wherein A is the top surface area of the multilayer structure.

The use of sufficiently thick micro-optics layers reduces total internalreflection and boosts light outcoupling.

According to an embodiment of the present invention a first layer, asseen from said semiconductor structure, of said multilayer structure hasa refractive index substantially equal to the refractive index of a topregion of said semiconductor structure. Thereby, an improved lightcoupling into the multilayer structure is achieved as the reduceddiscontinuity in the refractive index between the first layer and thesemiconductor structure reduces backward reflections. Light trappingwithin the light emitting diode component is thereby reduced. In anembodiment the refractive index of the multilayer structure deviatesless than 10% from the refractive index of a top region of saidsemiconductor structure. In another embodiment the refractive index ofthe multilayer structure deviates less than 5% from the refractive indexof a top region of said semiconductor structure. In yet anotherembodiment the refractive index of the multilayer structure deviatesless than 1% from the refractive index of a top region of saidsemiconductor structure.

According to an embodiment of the present invention the light emittingdiode component further comprises a wavelength converting layer. Thisembodiment provides means to adjust the spectral range of the lightgenerated by the light emitting diode component. In other words directlight, of a first wavelength range which is generated by thesemiconductor structure is converted by the wavelength converting layerinto light of a second wavelength range. The semiconductor structure andthe wavelength converting layer, being a single unit, further simplifiesthe assembly of light emitting diodes in arrays or other configurationsfor providing efficient illumination sources.

By wavelength converting layer is meant a layer comprising materialcapable of converting light of a first wavelength range into light of asecond wavelength range.

The wording direct light should be construed as light generated directlyby the semiconductor structure without any secondary optical processes.

According to an embodiment the wavelength converting layer comprises aphosphor material, a quantum dot, and/or a fluorescent dye. A phosphormaterial is, in context of the present invention, defined as a materialor substance exhibiting light emission after excitation in luminescence,fluorescence or phosphorescence processes.

According to another embodiment the phosphor material comprises apolycrystalline plate, preferable comprising Ce (III) doped gadoliniumaluminium garnet (Y, GdAG:Ce). An advantage being that thepolycrystalline ceramic plate of YAG:Ce has highly controllable opticaland geometrical properties which allows for improved performance of thelight emitting diode component. The ceramic plate has a materialstructure such that scattering within the material is low which allowsfor improved manufacturing of the light emitting diode component andfurther allows for a high package efficiency.

According to an embodiment of the present invention the multilayerstructure is arranged on top of said wavelength converting layer. It isthereby possible to provide efficient means to couple light out from thewavelength converting layer.

According to an embodiment of the present invention the multilayerstructure is arranged below said wavelength converting layer. It isthereby possible to reduce the amount of light from said wavelengthconverting layer being reflected towards said light emittingsemiconductor structure.

According to another embodiment of the present invention the lightemitting diode component further comprises an additional micro-opticalmultilayer structure, wherein said wavelength converting layer isarranged in-between said micro-optical multilayer structure and saidadditional micro-optical multilayer structure. It is thereby possible toprovide efficient means to couple light out from the wavelengthconverting layer and to reduce the amount of light from said wavelengthconverting layer being reflected back towards said light emittingsemiconductor structure.

According to another embodiment the light emitting diode componentfurther comprises a substrate, preferable a sapphire substrate. Anadvantage being that the substrate on top of which the light emittingsemiconductor structure is fabricated does not have to be removed. Thissimplifies manufacturing of the light emitting diode component whichallows for cost efficient production. A sapphire substrate is a low losstransparent material upon which it is for instance suitable toepitaxially grow GaN based semiconductor structures.

According to an embodiment of the present invention the light emittingdiode component further comprises a side layer arranged adjacent to aside surface of said semiconductor structure. The side layer providesbetter design flexibility when designing the light output of the lightemitting diode component.

According to another embodiment the side layer comprises a wavelengthconverting material, preferably a phosphor material, a quantum dot,and/or a fluorescent dye. This allows for enhanced side emission andthereby better light uniformity of the light emitting diode componentwhich may be of advantage in illumination applications such asbacklighting.

According to another embodiment of the present invention the side layercomprises a light reflecting coating material, preferably comprisinghigh reflectance metals or high diffuse reflectance fluoropolymers. Animproved light guidance of the light emitting diode component can herebybe obtained.

According to a second aspect of the present invention a light emittingdiode (LED) is provided, said LED comprising the above mentioned lightemitting diode component arranged on a sub-mount.

According to a third aspect of the present invention a multilayerstructure is provided wherein said multilayer structure is arranged toguide light out from a light emitting semiconductor structure having atop surface, said multilayer structure comprising a plurality of layerswherein an i+1:th layer is arranged on top an i:th layer in a sequenceas seen from said semiconductor structure, wherein a refractive index,n_(i), of the i:th layer is greater than a refractive index, n_(i+1), ofthe i+1:th layer.

It is noted that the invention relates to all possible combinations offeatures recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showingembodiments of the invention.

As illustrated in the figures, the sizes of layers and regions areexaggerated for illustrative purposes and, thus, are provided toillustrate the general structures of embodiments of the presentinvention. Like reference numerals refer to like elements throughout.

FIG. 1a is a schematic cross sectional view of an embodiment of a LEDaccording to the present invention.

FIG. 1b is a schematic cross sectional view of an alternative embodimentof a LED according to the present invention.

FIG. 1c is a schematic cross sectional view of yet an alternativeembodiment of a LED according to the present invention.

FIG. 1d is a schematic cross sectional view of a further alternativeembodiment of a LED according to the present invention.

FIG. 2 is a schematic cross sectional view of yet an alternativeembodiment of a LED according to the present invention.

FIG. 3a is a schematic cross sectional view of a further embodiment of aLED according to the present invention.

FIG. 3b is an alternative embodiment of a LED according to the presentinvention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which currently preferredembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided for thoroughness and completeness, and fully convey the scopeof the invention to the skilled person.

FIG. 1a shows a schematic cross sectional view of a LED 100 according tothe present invention. The LED comprises a light emitting diodecomponent 101 wherein a semiconductor structure 104 is arranged to emitlight and a micro-optical multilayer structure 102 is arranged to guidelight out from the semiconductor structure 104. The light emitting diodecomponent is further attached to a sub-mount 106. The micro-opticalmultilayer structure 102 comprises a plurality of layers 102 a -102 d.The i+1:th layer of said plurality of layers 102 a -102 d is arranged ontop the i:th layer in the sequence as seen from said semiconductorstructure 104. The value of i is selected from the set of positiveintegers, thus i can be 1, 2, 3, 4, etc. For example, in this embodimentthe 2^(nd) layer is provided on the 1^(st) layer, the 3^(rd) layer isprovided on the 2^(nd) layer and the 4^(th) layer is provided on the3^(rd) layer. A refractive index, n_(i), of the i:th layer is furthergreater than a refractive index, n_(i+1), of the i+1:th layer.Furthermore, a thickness of the i+1:th layer is larger than a thicknessof the i:th layer. This reduces the total internal reflection whichincreases the light extraction efficiency and thus improves the lightoutput performance. In embodiments the increase of the thickness of thei+1:th layer with respect to the thickness of the i:th layer depends onthe relative increase of the refractive index of the i+1:th layer withrespect to the i:th layer. For example a larger increase of therefractive index of the i+1:th layer with respect to the i:th layerresults in a larger increase of the thickness of the i+1:th layer withrespect to the thickness of the i:th layer. In a practical example thei+1:th layer has a refractive index of 1.65 the i:th layer has arefractive index of 1.55 and then the thickness of the i+1:th layer isabout 440 μm and the thickness of the i:th layer is about 360 μm. Anincreased thickness, optimized according to the refractive indices, maychange the angular field emission which may become broader which isbeneficial in many applications requiring side emission.

In a further embodiment, a thickness of the i:th layer, t_(i), is givenby:

$t_{i} = \frac{\sqrt{A}}{\tan \left\lbrack {\sin^{- 1}\left( \frac{n_{i + 1}}{n_{i}} \right)} \right\rbrack}$

wherein A is the top surface area of the micro-optical multilayerstructure 102. The surface area of the multilayer structure 102 issubstantially equal to the surface area of the top surface of the lightemitting semiconductor structure 104.

In FIG. 1a the semiconductor structure 104 is shown in a flip-chip (FC)configuration. The semiconductor structure 104 comprises an epitaxiallayer 104 a and a substrate 104 b. The epitaxial layer 104 a furthercomprises a pn-junction, having at least one n-type GaN layer, one GaNbased active region, and one p-type GaN layer. Light is generated in theactive region after biasing the n- and p-type layers. The generatedlight is extracted from the sides of the GaN layers, typically then-type layer, and substrate 104 b. The substrate 104 b is therefore ofsapphire which is a low loss transparent material. In addition, sapphirehas a lattice constant similar to that of GaN enabling good growthquality of the GaN epitaxial layer 104 a of GaN.

The semiconductor structure 104 is attached to the sub-mount 106. Then-type and p-type regions of the epitaxial layer 104 a can beelectrically contacted via metal layers (not shown) to metal contacts(not shown) in the sub-mount 106. The attachment of the semiconductorstructure 104 to the sub-mount may e.g. be made by means of stud-bumps.But also other attachment methods known by the skilled person could besuitable to use.

The sub-mount 106 is highly reflecting in order to reflect lightgenerated by the semiconductor structure and emitted in a directiontowards the sub-mount 106. This improves the light emission through thetop surface of the semiconductor structure 104. The sub-mount may e.g.be a highly reflective printed circuit board (PCB) having for instance aSMD like geometry.

It should be noted that other configurations of the semiconductorstructure is possible as will be described below.

The light emitting diode component 101 further comprises a wavelengthconverting layer 105. The use of a wavelength converting layer 105provides means to adjust the spectral range of the light generated bythe light emitting diode component 101.

According to this embodiment the light emitting diode component 101 isarranged to emit white light. Light in the blue spectral range isproduced by a GaN based epitaxial layer 104 a emitting direct blue lightand the wavelength converting layer 105 comprises a phosphor, here apolycrystalline plate comprising Ce (III) doped gadolinium aluminiumgarnet (Y, GdAG:Ce) generating light in the yellow spectral region.Hence, the light emitting diode component 101 emits both blue and yellowlight, which in combination provide white light. An advantage of thisset-up is that the polycrystalline ceramic plate of YAG:Ce has highlycontrollable optical and geometrical properties which allows forimproved performance of the light emitting diode 101. The ceramic platehas for instance a refractive index of about 1.8 in the visible range ofthe wavelength spectrum which closely matches that of sapphire. Therebya reduction of the scattering at the interface between the substrate 104b and the wavelength converting layer 105 can be obtained. Thewavelength converting layer 105 has further a material structure suchthat scattering within the material is low which allows for improvedlight emitting efficiency.

The micro-optical multilayer structure 102 comprises transparent layers102 a-102 d arranged as described above. By subsequently grading therefractive index of the layers, as seen from the semiconductor structure104, a large portion of the light generated within the semiconductorstructure 104 can be emitted from the light emitting diode component101. In other words, the light scattering is reduced at the interfacesof the layers and an overall increase in the light output from the lightemitting diode 101 is accomplished. By additionally applying acorresponding grading of the thickness of the layers, the light emittingefficiency of the light emitting diode 101 is further improved.

The micro-optical multilayer structure 102 is arranged on top of saidwavelength converting layer 105 being a top portion of the semiconductorstructure 104. An improved light coupling into the micro-opticalmultilayer structure 102 may be achieved by tailoring the refractiveindex of the first layer 102 a, as seen from the semiconductor structure104, of the micro-optical multilayer structure 102 and the top portionof the semiconductor structure 104 resulting in reduced light scatteringat the interface of the two. Light trapping within the semiconductorstructure 104 is thereby reduced.

In order to reduce the light scattering at the interface of the firstlayer 102 a, as seen from the semiconductor structure 104, of themicro-optical multilayer structure 102 and the top portion of thesemiconductor structure 104, the first layer 102 a of the micro-opticalmultilayer structure 102 has a refractive index substantially equal tothe refractive index of a top region of the semiconductor structure 104.In an embodiment the refractive index of the first layer 102 a of themicro-optical multilayer structure 102 deviates less than 10% from therefractive index of a top region of said semiconductor structure 104. Inanother embodiment the refractive index of the first layer 102 a of themicro-optical multilayer structure 102 deviates less than 5% from therefractive index of a top region of said semiconductor structure 104. Inyet another embodiment the refractive index of the first layer 102 a ofthe micro-optical multilayer structure 102 deviates less than 1% fromthe refractive index of a top region of said semiconductor structure104. In yet another embodiment the refractive index of the first layer102 a of the micro-optical multilayer structure 102 is equal to therefractive index of a top region of said semiconductor structure 104.

FIG. 1b shows a schematic cross sectional view of a LED 110 according toan alternative embodiment of the present invention. The LED comprises alight emitting diode component 101, wherein a semiconductor structure104 is arranged to emit light, a micro-optical multilayer structure 102,as described in relation to FIG. 1a , is arranged to guide light outfrom the semiconductor structure 104. A wavelength converting layer 105is further arranged on top of the micro-optical multilayer structure102. An advantage of this embodiment is that the micro-opticalmultilayer structure 102 can reduce the amount of light emitted from thewavelength converting layer being reflected back towards the lightemitting semiconductor structure. Hence an improved LED performance isobtained.

FIG. 1c shows a schematic cross sectional view of a LED 120 according toanother embodiment of the present invention. The light emitting diodecomponent comprises the same components as described in relation to FIG.1a and FIG. 1b and in addition an additional micro-optical multilayerstructure 102′. The micro-optical multilayer structure 102 and theadditional micro-optical multilayer structure 102′ are arranged onopposing sides of the wavelength converting layer 105. It is therebypossible to provide efficient means to couple light out from thewavelength converting layer and to reduce the amount of light from saidwavelength converting layer being reflected back towards said lightemitting semiconductor structure.

It should be noted that according to this embodiment the refractiveindices of the first layers 102 a, 102′a of the micro-optical multilayerstructure 102 and the additional the micro-optical multilayer structure102′ are equal to or slightly lower than the refractive index of thesemiconductor structure 104 and that of the wavelength converting layer105, respectively. According to the description above the refractiveindices of the layers 102 b, 102′b are lower than that of the firstlayers 102 a, 102′a.

Moreover, according to an embodiment of the present invention therefractive index of the final layer 102 d of the micro-opticalmultilayer structure 102 being arranged below the wavelength convertinglayer 105 as seen from the top of the light emitting diode component 101is lower than the refractive index of the wavelength converting layer105, which can have a refractive index similar to that of the epitaxiallayer 104 a or the substrate 104 b.

It should also be noted that the micro-optical multilayer structure 102and the additional micro-optical multilayer structure 102′ may beidentical, hence comprising the similar layers structures and havingsubstantially the same physical properties.

The micro-optical multilayer structure 102 may comprise standard glassof for instance SF 11 type. Alternatively, the multilayer structure 102may comprise epoxy or silicon materials. Highly transparent materials ofthis kind are readily available with refractive indices between at least1.48 and 2. Index matching glues to form the multilayer stack are alsoneeded and generally available in the market (see e.g. U.S. Pat. No.7,423,297B2). Alternatively, silicone based layers may be bonded bymeans of an over moulding process like proposed in U.S. Pat. No.7,452,737B2.

According to one embodiment a top layer of the micro-optical multilayerstructure has a refractive index higher than or equal to the refractiveindex of a medium being on top of the micro-optical multilayerstructure. The tailoring of the refractive index difference between thetop layer of the micro-optical multilayer structure and the surroundingmedium, which may for instance be air or silicone, provides improvedlight guidance and light output.

According to another embodiment of the present invention the lightemitting diode component 101 is enclosed in a dome shaped lens structure108, as shown in FIG. 1d . The lens structure is arranged to increasethe light output and direct the light from the semiconductor structure.The dome shaped lens structure 108 may for instance be made of silicone.

A top layer 102 d of the micro-optical multilayer structure 102 has arefractive index higher than or equal to the refractive index of amedium being on top of the top layer 102 d. The tailoring of therefractive index difference between the top layer 102 d of themultilayer structure 102 and the surrounding medium, i.e. air or thelens structure 108 provides improved light guidance and light output.

Light generated by the epitaxial layer is, in state of the art FCconfigurations such as in Patterned Sapphire Substrate (PSS) based lightemitting diodes, coupled out via a sapphire substrate into either theair or via the dome encapsulation material and then into the air. Hencea large portion of the generated light is trapped by total internalreflection in the light emitting diode component. The sapphire substrateis moreover typically 100 to 800 micrometers in thickness which leads toa large portion of the light being emitted in a direction towards thesides of the substrate. This light is to a large extent reflected at theinterface of the substrate and its surrounding medium due to the largerefractive index mismatch at the interface. To ensure that at least apart of this back reflected light can exit the light emitting diodecomponent, the highly reflective sub-mounts are commonly used in FCbased light emitting diode components.

According to an embodiment of the present invention it is an objectiveto mitigate this problem.

Accordingly, FIG. 2 shows a schematic cross sectional view of a LED 200according to the present invention. The LED 200 comprises a lightemitting diode component 101 comprising a semiconductor structure 104and a micro-optical multilayer structure 102 as disclosed in relation toFIG. 1a to FIG. 1d . The light emitting diode component 101 furthercomprises a side layer 104 d arranged adjacent to a side surface of saidsemiconductor structure 104. The side layer 104 d covers one or more ofthe sides of the semiconductor structure 104. The side layer providesbetter design flexibility when designing the light output of the lightemitting diode component 101. The side layer 104 d comprises wavelengthconverting material, preferably a phosphor material. The side layer 104d and the wavelength converting layer 105 may form a common layer. Theside layer 104 d and the wavelength converting layer 105 may be arrangedto provide light emission having the same colour point.

It should be noted that the side layers 104 d extend from the epitaxiallayer 104 a of the semiconductor structure 104 at least to thewavelength converting layer 105. Hence a large portion of the lightbeing emitted in a direction towards the sides of the substrate canthereby enter the side layer 104 d, where the spectral range of at leasta portion of the light can be changed by the wavelength convertingmaterial of the side layer 104 d. An overall improved light emission isthereby obtained. Moreover, the light emitted by the light emittingdiode component 101 may have an increased angular distribution which canbe of advantage for example in backlighting applications where theperceived light spottiness of the light emitting diodes are to bereduced.

Alternatively, the side layer 104 d may comprise a light reflectingmaterial arranged to reflect light emitted from the semiconductorstructure 104, i.e. from the epitaxial layer 104 a and the wavelengthconverting layer 105 so that a larger portion of the emitted light iscoupled out of the top layer 102 d of the light emitting diode component101.

According to another embodiment of the present invention the substrateis at least partly removed. The removal of at least a part of thesubstrate may e.g. be done by laser assisted lift-off, grinding,chemical- mechanical polishing, or wet etching or any other suitableprocessing technique. The removal of at least a part of the substratemaybe made such that the semiconductor structure is at least partlyexposed. The resulting device structure is usually referred to as alight emitting diode having a thin-film-flip-chip (TFFC) geometry. Withthe TFFC geometry absorption losses in the transparent substrate can beavoided providing improved brightness of the light emitting diodecomponent. A further increase in the light output of the light emittingdiode may be achieved by roughening or patterning of the exposed surfaceof the semiconductor structure whereby the fraction of light coupled outfrom the semiconductor structure improved. As a result the externalquantum efficiency of the light emitting diode component may beimproved. A further advantage is that other substrates may be usedduring epitaxial growth, including silicon or SiC.

FIG. 3a discloses a light emitting diode (LED) 300, comprising a lightemitting diode component 101 having a TFFC geometry. The light emittingdiode component 101 comprises a semiconductor structure 104 comprisingan epitaxial layer 104 a and a wavelength converting layer 105, asdescribed above. The epitaxial layer 104 a is mounted on a sub-mount 106and the wavelength converting layer 105 is deposited on the epitaxiallayer 104 a. A micro-optical multilayer structure 102 comprising aplurality of layers 102 a-102 d is arranged on top of the wavelengthconverting layer 105.

The light emitting diode component 101 further comprises a side layer104 d arranged adjacent to a side surface of said semiconductorstructure 104. The side layer 104 d covers one or more of the sides ofthe semiconductor structure 104. The side layer 104 d comprises lightreflecting material arranged to reflect light emitted from thesemiconductor structure 104, i.e. from the epitaxial layer 104 a and thewavelength converting layer 105 so that a larger portion of the emittedlight is coupled out of the top layer 102 d of the light emitting diodecomponent 101.

Additionally, the side layer 104 d may comprise wavelength convertingmaterial, preferably a phosphor material. The function and benefits ofusing converting material are described above.

FIG. 3b discloses an alternative embodiment of a LED 310, comprising alight emitting diode component 310 having a TFFC geometry. The lightemitting diode component 101 comprises a semiconductor structure 104comprising an epitaxial layer 104 a, a side layer 104 d and a wavelengthconverting layer 105, as described above. The light emitting diodefurther comprises a micro-optical multilayer structure 102 and anadditional micro-optical multilayer structure 102′ arranged on opposingsides of the wavelength converting layer 105.

The advantages of the present embodiment are the same as the onesdescribed in relation to FIG. 1c. The side layers 104 d extend from theepitaxial layer 104 a at least to the wavelength converting layer 105 toprevent direct light emitted from the epitaxial layer 104 a to exit thesides of the micro-optical multilayer structure 102.

In case the side layer 104 d comprises a reflecting material this leadsto a large portion of the light being emitted in a direction towards thesides of the multilayer structure 102 can be reflected towards thewavelength converting member 105.

In case the side layer 104 d comprises a wavelength converting memberthe large portion of the light being emitted in a direction towards thesides of the multilayer structure can enter the side layer 104 d, wherethe spectral range of at least a portion of the light can be changed bythe wavelength converting material of the side layer 104 d. Hence anoverall improved light emission is obtained. Moreover, may the lightemitted by the light emitting diode component 101 have an increasedangular distribution which can be of advantage for example inbacklighting applications where the perceived light spottiness of thelight emitting diodes are to be reduced.

Computer simulations reveal that using a micro-optical multilayerstructure according to the present invention result in improved lightoutput from light emitting diode components. The light output from twodifferent light emitting diode components were compared with a state ofthe art PSS chip scale package (CPS) LED. The PSS-CPS LED comprise asemiconductor structure having a substrate layer with a refractive indexof 1.8, and top and side layers each having a refractive index of 1.5.

By increasing the refractive index of the wavelength converting layer tobe substantially the same (about 1.8) to that of the substrate layer, byfor instance using a high refractive index material such as a wavelengthconverting layer comprising Ce (III) doped gadolinium aluminium garnet(Y, GdAG:Ce), the relative gain in the light output was simulated to be3.4%. If additionally a multilayer layer structure is formed byarranging a transparent layer having a refractive index of 1.7 on top ofthe high refractive index material a relative gain in the light outputof about 7.6% can be achieved. Moreover, a relative gain in light outputof 12.1% can be achieved if a multilayer structure comprising a firstlayer and a second layer on top of the high refractive index material isused. The first and the second layer were set to have a refractive indexof 1.7 and 1.48, respectively.

For simplicity, each the layers were assumed to have a thickness of 400micrometer. Further light emitting efficiency improvements may beobtained by a grading of the thickness of subsequent layerscorresponding to the grading of the refractive index of the subsequentlayers. Thus, an increase of the refractive index between subsequentlayers corresponds to an increase of the thickness between subsequentlayers. Analogously, a decrease of the refractive index betweensubsequent layers corresponds to a decrease of the thickness betweensubsequent layers.

The simulations also show that encapsulated light emitting diodes canhave an increased light output when comprising a multilayer structure.In the simulations a 3 mm lens structure having a refractive index of1.5 (e.g. silicone) was used. The first and the second layer were set tohave a refractive index of 1.75 and 1.6, respectively. The gainsachieved for the different configurations disclosed above were for theencapsulated light emitting diodes 7.0%, 8.3%, and 8.5%, respectively.

Hence it should be noted that according to the simulations a lightemitting diode component comprising a multilayer structure but beingwithout encapsulation may outperform a reference PSS-CSP LED structurewith a lens structure. This insight may be advantageous when fabricatinghighly efficient light emitting diodes and further allow for increasedpacking efficiency of the light emitting diodes.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims.

For example the wavelength converting layer 105 may comprise a quantumdot (QD). QDs are small crystals of semiconducting material generallyhaving a width or diameter of only a few tens of nanometers. They havethe advantage that when excited by incident light, they emit light wherethe wavelength of the light is determined by the size and material ofthe QD. Further, they show very narrow emission bands and thus providesaturated colors, where the light output of a particular color can beproduced by tailoring the material and size of the QDs used. QDs withemission in the far-red upon red excitation could for instance beachieved by using QDs comprising a material selected from the groupconsisting of but not limited to II-VI and III-V QDs, preferably InP,CdTe, CdTe/CdSe core-shell structures, ternary mixtures such asCdSe_(x)Te_(y), or chalcopyrite QDs such as Cu_(x)In_(y)Se₂ orCu_(x)In_(y)S₂. The QDs can be overcoated with higher band-gap materialssuch as CdS and ZnS for enhanced emissive properties.

The wavelength converting layer 105 could comprise an inorganicphosphor, wherein said inorganic phosphor comprises a material dopedwith Cr³⁺, preferably a material selected from the group consisting ofY₃A₁₅O₁₂:Cr³⁺.

The wavelength converting layer 105 could comprise a fluorescent dye.

The wavelength converting layer 105 may comprise Lumiramic™

For example the active region of the epitaxial layer 104 a can furtherinclude heterostructure layers comprising e.g. In_(x)Al_(y)G_(1-x-y)N totailor the band gap and thereby the emission wavelengths of the lightemitting diode component. Quantum well (QW) or multi-quantum well (MQW)structures, obtained using for instance quantum confining layers ofGaIn/In_(x)G_(1-x)N could further be located in the active region tolocally increase the concentration of holes and electrons, which due tothe increased recombination rate leads to an increased number of photonsemitted from the light emitting diode component.

The GaN layers may have a total thickness of about 5 micrometer. Thisthickness is not critical and could vary as long as high concentrationsof defects that presumably hinder the LED performance can be avoided.The sapphire substrate is typically 200 micrometers thick but in otherembodiments the thickness could range from 10-800 micrometers.

The light emitting diode component may further comprise bonding layersarranged to connect the plurality of layers in the multilayer structure.It is preferred that the bonding layers have refractive indexes matchingthe surrounding layers in order to reduce light scattering at theinterfaces.

The bonding layer may comprise silicone.

The lens structure may have various geometrical shapes such asspherical, or elliptical. The top of the lens structure may further betextured, randomly, into a Fresnel lens shape, or with a photoniccrystal structure.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage.

1. A method comprising: providing a light emitting semiconductorstructure grown on a growth substrate; removing at least a portion ofthe growth substrate; and forming a multilayer structure arranged toguide light out from a surface of the light emitting semiconductorstructure, said multilayer structure comprising a plurality of layers,wherein an i+1:th layer is arranged on top an i:th layer in a sequenceas seen from said light emitting semiconductor structure, wherein arefractive index, n_(i), of the i:th layer is greater than a refractiveindex, n_(i+1), of the i+1:th layer wherein the value of i is selectedfrom the set of positive integers, wherein a thickness of the i+1:thlayer is greater than a thickness of the i:th layer.
 2. The method ofclaim 1 wherein the thicknesses of the i+1:th layer and the i:th layerare on the order of hundreds of microns.
 3. The method of claim 1wherein a thickness of the i:th layer, t_(i), is given by:t_i=√A/tan[sin̂(−1)(n_(i+1)/n_i)] wherein A is the top surface area ofthe multilayer structure.
 4. The method of claim 1 further comprisingattaching the light emitting semiconductor structure to a mount.
 5. Themethod of claim 1 wherein removing at least a portion of the growthsubstrate comprises removing the entire growth substrate.
 6. The methodof claim 5 further comprising roughening or patterning a surface of thelight emitting semiconductor structure exposed by removing the entiregrowth substrate.
 7. The method of claim 5 wherein forming a multilayerstructure comprises forming the multilayer structure on a surface of thelight emitting semiconductor structure exposed by removing the entiregrowth substrate.
 8. The method of claim 5 further comprising disposinga wavelength converting material on a surface of the light emittingsemiconductor structure exposed by removing the entire growth substrate.9. The method of claim 1 wherein removing at least a portion of thegrowth substrate comprises a technique selected from the groupconsisting of laser-assisted lift off, grinding, chemical-mechanicalpolishing, and wet etching.
 10. The method of claim 1 further comprisingforming a wavelength converting material over the light emittingsemiconductor structure.
 11. The method of claim 1 further comprisingforming a reflective material on a side surface of the light emittingsemiconductor structure.
 12. The method of claim 1 further comprisingforming a lens over the light emitting semiconductor structure.
 13. Adevice comprising: a light emitting semiconductor structure attached toa mount; and a multilayer structure disposed over a surface of the lightemitting semiconductor structure from which at least a portion of agrowth substrate has been removed, the multilayer structure comprising aplurality of layers, wherein an i+1:th layer is arranged on top an i:thlayer in a sequence as seen from said semiconductor structure, wherein arefractive index, n_(i), of the i:th layer is greater than a refractiveindex, n_(i+1), of the i+1:th layer wherein the value of i is selectedfrom the set of positive integers, wherein a thickness of the i+1:thlayer is greater than a thickness of the i:th layer.
 14. The device ofclaim 13 wherein the multilayer structure is disposed in direct contactwith the surface of the light emitting semiconductor structure fromwhich at least a portion of a growth substrate has been removed.
 15. Thedevice of claim 13 further comprising a wavelength converting materialdisposed between the multilayer structure and the surface of the lightemitting semiconductor structure from which at least a portion of agrowth substrate has been removed.
 16. The device of claim 15 furthercomprising an additional multilayer structure, wherein said wavelengthconverting layer is arranged in-between said multilayer structure andsaid additional multilayer structure.
 17. The device of claim 13 furthercomprising a lens disposed over the multilayer structure.
 18. The deviceof claim 13 wherein a first layer, as seen from said light emittingsemiconductor structure, of the multilayer structure has a refractiveindex equal to the refractive index of a top region of said lightemitting semiconductor structure.
 19. The device of claim 13 furthercomprising a side layer arranged adjacent to a side surface of saidsemiconductor structure.
 20. The device of claim 19, wherein said sidelayer is selected from the group consisting of a wavelength convertingmaterial, a phosphor material, a quantum dot, a fluorescent dye, a lightreflecting coating material, high reflectance metals, and high diffusereflectance fluoropolymers.